Hyaluronic acid (also called Hyaluronan or sodium hyaluronate), abbreviated as HA, is a non-sulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is one of the chief components of the extracellular matrix and contributes significantly to cell proliferation and migration. HA may also be involved in the progression of some malignant tumors. The average 70 kg man has roughly 15 grams of HA in his body, one third of which is degraded and synthesised every day.
Lubricants for Joints
HA is a major component of the synovial fluid and is found to increase the viscosity of the synovial fluid. Along with lubricin, it is one of the main lubricating components of synovial fluid. HA is an important component of articular cartilage, in which it coats each cell (chondrocyte). When aggrecan monomers bind to HA in the presence of link protein, large highly negatively charged aggregates are formed. These aggregates imbibe water and are responsible for the resilience of cartilage (its resistance to compression). The molecular weight (size) of HA in cartilage decreases with age, however the amount of HA in cartilage increases with age.
Biomatrix Components for Skin Tissue Repair
HA is a major component of skin, where it is involved in tissue repair. When skin is excessively exposed to UVB rays, the HA present in skin acts as a free radical scavenger, absorbing free radicals. The skin becomes inflamed (sunburn) and the cells in the dermis stop producing as much HA. The rate of its degradation is also increased. HA degradation products also accumulate in the skin after UV exposure.
While it is abundant in extracellular matrices, HA also contributes to tissue hydrodynamics, movement and proliferation of cells, and participates in a number of cell surface receptor interactions, notably those including its primary receptor, CD44. Upregulation of CD44 itself is widely accepted as a marker of cell activation in lymphocytes. HA's contribution to tumor growth may be due to its interaction with CD44. CD44 participates in cell adhesion interactions required by tumor cells. Although HA binds to CD44, there is evidence showing that HA degradation products transduce their inflammatory signal through Toll-like receptor 2 (TLR2), TLR4 or both TLR2 and TLR4 in macrophages and dendritic cells. TLR and HA play a role in innate immunity.
Structure and Synthesis of HA
The structure of HA is well characterized. It is composed of repeated units of disaccharide of D-glucuronic acid and D-N-acetylglucosamine, linked together via alternating β-1,4 and β-1,3 glycosidic bonds. HA is typically up to 25,000 disaccharide units in length. The molecular weight of HA can range from 5,000 to 20,000,000 in vivo. The average molecular weight in human synovial fluid is 3 to 4 million and HA purified from human umbilical cord is typically 3,140,000 Da.
HA is synthesized by a class of integral membrane proteins called HA synthases, of which vertebrates have three types: HAS1, HAS2, and HAS3. These enzymes lengthen HA by repeatedly adding glucuronic acid and N-acetylglucosamine to the nascent polysaccharide as it is extruded through the cell membrane into the extracellular space. Commercially, HA can be produced from animal sources, for example, from rooster combs, umbilical cords, and from the cartilage of joints. Alternatively HA may be obtained through fermentation. The average molecular weight of HA varies according to the source from which it is obtained. Generally the molecular weight of HA is 60,000 to 14,000,000.
Commercially available HA is generally produced from animal sources or from bacterial sources. Generally HA is obtained from bacterial sources through fermentation processes. The fermentation processes involve many steps including HA synthesis, separation and purification. The purity of HA is dependent on the source it is obtained from, in particular it is dependent on the bacteria used and the process techniques. The purity of HA has a significant influence on its stability.
Biocompatibility and Medical Applications of HA
HA is nontoxic, non-immunogenic and biodegradable. HA is degraded by a family of enzymes called hyaluronidases. In humans, there are at least seven types of hyaluronidase-like enzymes, several of which are tumor suppressors. The degradation products of HA, the oligosaccharides and very low molecular weight HA, exhibit pro-angiogenic properties. In addition, recent studies have shown that HA fragments, rather than high molecular HA components, can induce inflammatory responses in macrophages and dendritic cells in tissue injury and in skin transplant rejection.
The first HA biomedical product, Healon, was developed in the 1970s and 1980s and is approved for use in eye surgery (i.e., corneal transplantation, cataract surgery, glaucoma surgery and surgery to repair retinal detachment). Other biomedical companies also produce brands of HA for ophthalmic surgery.
HA is also used to treat osteoarthritis of the knee. Such treatments, called viscosupplementation, are administered as a course of injections into the knee joint and are believed to supplement the viscosity of the joint fluid thereby lubricating the joint, cushioning the joint and producing an analgesic effect. It has also been suggested that HA has positive biochemical effects on cartilage cells. The molecule weight is varied from 750,000 to 2 million and some of the preparation in which HA is lightly crosslinked. However, some placebo controlled studies have cast doubt on the efficacy of HA injections, and HA is recommended primarily as a last alternative to surgery. Oral use of HA has been lately suggested although effectiveness still needs to be demonstrated. Some preliminary clinical studies suggest that oral administration of HA has a positive effect on osteoarthritis.
Due to its high biocompatibility and its common presence in the extracellular matrix of tissues, HA is gaining popularity as a biomaterial scaffold in tissue engineering research.
In some cancers, HA levels correlate well with malignancy and poor prognosis. HA is thus often used as a tumor marker for prostate and breast cancer. It may also be used to monitor the progression of the disease.
HA may also be used postoperatively to induce tissue healing, notably after cataract surgery. Current models of wound healing propose that larger polymers of HA appear in the early stages of healing to physically make room for white blood cells, which mediate the immune response.
Personal Lubricant
A natural acidic lubricating fluid is normally present in the vagina and during sexual arousal an increased amount of the fluid is produced. The function of production of sufficient vaginal lubrication may be impaired, causing vaginal tissue to become dry and irritated due to a number of causes including decreased estrogen levels during the menopause or after surgical removal of ovaries and after radiation therapy. This may result in pain during sexual intercourse and/or bleeding during sexual intercourse. Oral contraceptives and certain medications such as antihistaminics, antidepressants, blood pressure, and cardiac medicines can also contribute to vaginal dryness. Additionally, psychological conditions including stress, fatigue and anxiety may impede production of the natural lubricant. A combination of hormonal and psychological factors may induce dryness temporarily after childbirth particularly if the mother is breastfeeding. Stress for those couples under IVF treatment for conceiving is another particular example.
Common personal lubricants such as silicone and those sold under the Trade Marks K-Y Jelly® and Vaseline® are mainly based on synthetic substances such as silicone, petroleum. Commercially available lubricants containing glycerin are spermicidal and impede sperm motility. Even at low concentrations ingredients such as glycerin are associated with such spermicidal effects. Accordingly, such commercially available vaginal lubricants are not recommended for women seeking to conceive.
Personal lubricants comprising HA as the lubricating component are known for the treatment and relief of vaginal dryness.
HA has been found to be useful in IVF treatment. It is a normal component of mammalian follicular, oviductal, and uterine fluids (Lee and Ax 1984, Suchanek et al 1994, Rodriguez-Martinez et al 1998). Physiological concentrations of hyaluronan in these fluids, for instance, range from 0.04 to 1.83 mg/mL, 16 to 39% of all glycosaminoglycans (Kano et al 1998), which in turn provides a high viscosity environment in the oviduct and uterus. In vivo, it is known that hyaluronan supports ovulation and assists in sperm selection during the fertilization process.
Commercially available HA is a natural biodegradable polymer. It is especially unstable under extreme conditions-such as high temperature, high pH or low pH (alkaline and acidic condition), or in the presence of free radicals or free radical precursors. Under such conditions HA having a relatively high molecular weight, for instance 1 to 3 million tends to degrade into small HA fragments. In addition, the presence of impurities such as trace heavy metals like Fe2+, Fe3+, Cr3+, Cu2+, Al3+ etc will accelerate the degradation process.